In manufacturing electronic devices, there is performed a process of forming an opening such as a hole or a trench at a region of silicon oxide (SiO2). In such a process, a target object is exposed to a plasma of a fluorocarbon gas and then, the region of silicon oxide is etched, as disclosed in U.S. Pat. No. 7,708,859.
There is known a technique for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride. A SAC (Self-Aligned Contact) technique disclosed in Japanese Patent Application Publication No. 2000-307001 is known as an example of such a technique.
The target object to be processed by the SAC technique includes a first region of silicon oxide, a second region of silicon nitride, and a mask. The second region is formed to have a recess. The first region is formed to fill the recess and cover the second region. The mask is provided on the first region and has an opening provided over the recess. As disclosed in Japanese Patent Application Publication No. 2000-307001, in the conventional SAC technique, a plasma of a processing gas including a fluorocarbon gas, an oxygen gas and a rare gas is used to etch the first region. By exposing the target object to the plasma of the processing gas, a portion of the first region which is exposed through the opening of the mask is etched. As a consequence, an upper opening is formed. Further, by exposing the target object to the plasma of the processing gas, a portion surrounded by the second region, i.e., the first region in the recess, is etched in a self-aligned manner. Accordingly, a lower opening continuous to the upper opening is formed in a self-aligned manner.
In the above-described conventional technique, at a time when the second region is exposed in the course of etching the first region, there occurs a state where a film for protecting the second region is not formed on a surface of the second region. Thus, if the etching of the first region is further performed in this state, a portion of the second region is also etched.